A recently unearthed fossil has captured the moment an ichthyosaur – an ancient marine reptile – gave birth. The researchers involved in the latest study claim the remains show the earliest live birth, from an ancient Mosozoic marine reptile, perfectly preserved as a 248 million-year-old fossil collected from China. The study was published in the Feb. 12 issue of the journal PLOS ONE, entitled Terrestrial Origin of Viviparity in Mesozoic Marine Reptiles Indicated by Early Triassic Embryonic Fossils.
Ichthyosaurs were giant marine reptiles that first swam the seas some 250 million years ago, sharing the waters with other enormous sea creatures, including plesiosaurs and mosasaurs. Ichthyosaurs, which have also been dubbed sea monsters, originally possessed long, flexible bodies and soared through the water by undulation – an action similar to that witnessed in streamlined eels. Growing up to the length of a bus, the more advanced ichthyosaurs had fish-like bodies and crescent-shaped tails, and were primarily built for swimming at great speeds. The air-breathing carnivores are believed to have evolved from terrestrial reptiles that made their transition into the water, during the early Triassic.
The ichthyosaurs identified within the fossilized specimen belonged to the genus Chaohusaurus (Reptilia, Ichthyopterygia). The remains show an ichthyosaur baby, preserved inside its mother and another wedged in her pelvis, with just its skull emerging from the maternal parent. A third embryo was identified nearby, suggesting it was stillborn. Based upon the discovered remains, the research group conjecture the labor to have been complicated by unknown factors, ultimately, resulting in the mother’s demise.
All of the fossil specimens were obtained from a fossil quarry in south Majiashan, China, through a collaborative endeavor between Anhui Geological Museum, Peking University, University of California, Università degli studi di Milano and the Field Museum.
Startlingly, the new research study provides evidence to refute the contention that early ichthyosaurs gave birth in the water. The authors established that the ancient ichthyosaur’s offspring surfaced head-first from the birth canal – a behavior that is only observed in land-based animals. On this basis, the researchers conclude that the specimen demonstrates a terrestrial origin of viviparity.
This discovery came as quite a shock to the researchers, since most other air-breathing marine creatures – including dolphins, whales and sea cows (a.k.a. sirenians) – give birth to their offspring tail-first; this helps prevent the nascent creatures from suffocating during birth. Speaking to the National Geographic, Ryosuke Motani, study leader and expert of prehistoric marine reptiles at the University of California, described the team’s findings as a “big surprise.” Generally, reptiles tend to lay eggs, while mammals give birth to live young (viviparity).
Initial assumptions suggested that live births developed in marine reptiles after they had taken to the seas. However, Motani and colleagues have concluded, based upon the labor revealed in the latest ichthyosaur fossil, live birth in marine reptiles is linked to terrestrial-style birth. During a recent press release, Dr. Motani stresses that his team’s research findings suggests “… live-bearing evolved on land and not in the sea.”
In addition, the new Chaohusaurus fossil specimen may contain the oldest fossilized embryos of Mesozoic marine reptiles and is around 10 million years older than other, previously reported, fossilized embryos from such reptiles.
By James Fenner
Top image credit: R. Motani, PLOS ONE